What percentage of a tile can lack adhesive
How much adhesion a tile can lack – and what it means in practice
When a building surveyor inspects a property, hollow or loose tiles are among the most common findings in a condition report. This raises a natural question: is it a serious problem, or does it fall within normal tolerance?
The answer depends on how much of the tile's underside lacks contact with the substrate – and there is in fact a recognised rule of thumb for how much adhesion a tile is allowed to lack.
The 20% rule: what counts as an acceptable void
Within the tiling industry, 20% is the widely accepted limit for permissible voids beneath a tile – and thus for how much of a tile may lack adhesion. Up to one-fifth of the tile's area may lack contact with the substrate, provided the tile is otherwise firmly seated and does not yield under load.
In concrete terms: a tile measuring 30 × 60 cm has an area of 1,800 cm². 20% equals 360 cm² – roughly an 18 × 20 cm patch without contact to the substrate. Within that limit, the tile is considered functional.
The rule applies primarily to tiles on floors and walls in standard environments. In a Danish context, it is sometimes referred to as a tile-installation norm for minimum adhesion, although no single codified standard exists. In high-load areas – stair landings, public floors, wet rooms with frequent water exposure – tolerance is lower, and many professionals recommend full contact or close to it.
The tap test: how to find hollow and loose-sounding tiles
One way to identify hollow tiles is the tap test: tap lightly with a coin or knuckle across the tile's surface and listen to the sound. A hollow, knocking sound – the hallmark of what professionals call hollow-sounding tiles – indicates an air gap beneath the tile. A firm, dull sound means good adhesion.
The tap test is not precise enough to determine whether the void exceeds the tolerance threshold for hollow tiles. It tells you that a void exists – and whether it is located at the edge, the centre or spread across the tile. A tile that sounds hollow only along the edge is likely within the limit. A tile that sounds hollow across most of its surface probably exceeds it.
When voids beneath tiles become a problem
Voids beneath tiles are not automatically dangerous – but they can become so if ignored. Lack of adhesion in tiles is not always visible, which is why the tap test matters as ongoing inspection. Three situations exist in which the 20% rule is insufficient as the sole evaluation criterion:
The tile is movable. If the tile rocks, creaks or yields under pressure, adhesion is insufficient – regardless of void size. Tiles without contact to the substrate are exposed to mechanical stress every time they are walked on, and cracks or detachment are only a matter of time.
The void is located at the edge. Edge voids are more critical than central ones, because the grout joint provides no structural support. Moisture ingress at the edge can also cause the bonding material to break down further.
The tile sits in a wet room. In bathrooms, shower areas and other wet rooms, tight adhesion is essential to prevent water penetrating behind the surface. Here, even minor voids should be taken seriously – wet rooms should always have 100% adhesion.
In all other cases – tiles that sound hollow but sit firmly and do not yield – the 20% rule is a reasonable starting point for assessment.
Repair instead of replacement
A common misconception is that loose tiles must always be replaced. This is rarely necessary. Modern injection methods make it possible to restore adhesion beneath hollow tiles without removing them.
FAQ – Frequently asked questions about tile adhesion
What does the standard say about insufficient adhesion beneath tiles?
The industry generally operates with a 20% limit for voids beneath tiles – meaning the accepted void beneath a tile is up to one-fifth of the tile's underside.
In demanding environments such as wet rooms, stairs and heavily loaded floors, however, a lower tolerance for voids and lack of adhesion is recommended.
What does it mean if a condition report mentions loose tiles?
Loose tiles are a common finding in a condition report and not necessarily a sign of serious damage.
The report will typically include a remark, however, if more than 20% of the tile sounds hollow, or if even minor voids occur in wet rooms, where adhesion requirements are higher.
What causes voids beneath tiles?
The causes are typically insufficient adhesive coverage during installation, drying of the adhesive layer, temperature fluctuations that create movement in the substrate, or settlement in the building over time.
Voids beneath tiles are therefore not necessarily a sign of faulty installation, but can develop gradually as a result of ordinary loads and movement in the structure.